#app4{

}
@keyframes change {
    0%{
        background: green;
    }
    100%{
        background: yellow;
    }
}
#app4 .circle {
    width: 20vw;
    height: 20vw;
    border-radius: 50%;
    border: 1px solid red;
}
#app4 .circle.active {
    animation: change 1s infinite linear alternate;
}
